Today, international travel has become commonplace. It is important for travelers to ensure drinking water at their travel destinations. Commercially available mineral waters are convenient drinking water for travelers. However, travelers unfamiliar with hard water may become ill with hard water because most of the mineral waters available in Europe, etc. are hard water. It is therefore convenient for travelers to have a portable device that can regulate the hardness of drinking water.
Water softening devices for reducing water hardness have been developed (see JP 2003-117550 A, JP 2003-334549 A, etc.). Conventionally developed water softening devices soften water using ion-exchange resins. In the water softening devices using the ion-exchange resins, sodium ion bonded to ion-exchange groups in the ion-exchange resins is replaced with calcium ion or magnesium ion in water. As a result, calcium ion and magnesium ion are adsorbed by the ion-exchange resins, and thus the water is softened. However, the ion-exchange resins have a limitation in their ion-exchange capacity. Accordingly, ion-exchange resins that have adsorbed a certain amount of calcium ion and magnesium ion need to be regenerated using sodium chloride water.
The conventional water softening devices using the ion-exchange resins have a problem in that they are difficult to be miniaturized and are not suitable for carrying. Moreover, the conventional water softening devices using the ion-exchange resins have a problem in that the regeneration of the ion-exchange resins takes a relatively long time. Moreover, the conventional water softening devices using the ion-exchange resins have a problem in that sodium chloride water is required to regenerate the ion-exchange resins.